1. Field of the Invention
The present invention relates to a data display radio pager (simply referred to as a pager hereinafter) and, more particularly, to a pager matching a transmission format based on a POCSAG standard system or similar system and including an address codeword made up of a plurality of address bits for designating a called pager and a plurality of function bits for specifying particular alert modes of a called pager, e.g., a tone mode and a message display mode.
2. Description of Prior Art
The POCSAG signal format which the pager of the the present invention typically matches has a 576-bit preamble for bit synchronization, 32-bit synchronization codewords for word synchronization and eight frames each being implemented as a pair of 32-bit codewords. The preamble and frames are transmitted in a batch. In a frame whose first bit (flag bit) is (logical) "1", the second to nineteenth bits, i.e., eighteen bits in total (2/19) serve as address bits which distinguish a called pager from the others. The twentieth and twenty-first bits (20/21) following such eighteen bits are the function bits while the twenty-second to thirty-second bits, i.e., eleven bits in total (22/32) are the check bits for checking transmission errors. A message codeword follows such a 32-bit address codeword. Up to fifteen codewords (7.5 frames) can be transmitted in a single batch with ONE set in their flag bits.
When the POCSAG standard system was in its infancy, the function bits (20/21) were used as call designation bits for designating one of four tones (A call, B call, C call and D call) each being a particular combination of ON and OFF durations of an audible frequency signal (see British Telecom Radio Paging: Specification RC4517 for Tone Page Receivers, Paragraph 5.1). Today, improved and inexpensive liquid crystal display (LCD) devices implement the combined use of alert tone and LCD display. As a result, the function bits are used to specify various alert modes including an alphanumeric message display mode, a numeric message display mode, an alphanumeric preformed message display mode, and a tone only display mode.
On the other hand, a pager of the type described receives a radio frequency (RF) signal from a central station which has been modulated by a digital paging signal made up of the address codeword and a message codeword that follows the address codeword. The pager demodulates the received RF signal to reproduce the digital paging signal and then separates the address codeword and message codeword from the demodulated signal. If the address signal coincides with an address signal assigned to the pager and stored in a memory built in the pager, the pager alerts the user of the pager by, for example, tone, alphanumeric message display or alphanumeric preformed message display as specified by the function bits of the address codeword.
It is a common practice with the above-described type of pager to define a relation between the plurality of alert modes and the function bits by writing codes representative of four kinds of modes corresponding one-to-one to the function bits "00", "01", "10" and "11" in a predetermined area of the memory before shipment from the manufacturer's factory.
However, capabilities available with conventional pagers are limited considering the fact that the subscribers of paging services, i.e., needs are increasingly diversifying. Regarding doctors or lawyers, for example, personal needs will be almost satisfied if the toner only mode and the alphanumeric message display mode are available. However, salesmen of stock companies, for example, need as much alphanumeric message display and preformed alphanumeric message display as possible rather than tone. Since the company operating a paging service can grasp such versatile needs more accurately than the manufacturer, the tendency that the manufacture entrusts the service operating company with the writing of alert mode designation codes in the memory is increasing. More specifically, there is an increasing tendency that the company operating a paging service writes not only the address codeword but also the alert mode designation codes in the memory to meet the varying demands flexibily.
Under such circumstances, it is often required to rewrite the codes stored in the memory built in the pager. However, the conventional pager using a PROM as stated above cannot have the codes thereof rewritten with ease.